Several polypeptide factors present in the thymus gland have been implicated to play an important role in the development and maintenance of immunological competence in man and in animals. The importance of the immune system in the defense against cancer and tumor cells is now widely recognized. In recent years, a few polypeptides shown to be able to stimulate maturation, differentiation and function of T cells have been isolated from bovine thymus. Among them, the peptide thymosin .alpha..sub.1 has been intensively studied. Its structure and activity have been described in U.S. Pat. No. 4,079,127.